Progress 10/01/11 to 09/30/12
Outputs
Progress Report Objectives (from AD-416): The objective of this collaborative project is to develop, process, evaluate, and simulate value-added processing operations for low-value biofuels byproducts. Approach (from AD-416): 1. Physically and chemically modify the material properties of low-value components of various biofuels byproduct streams in order to facilitate their conversion into value-added industrial products. 2. Examine industrial processes (such as injection and compression molding) to convert low-value components of various biofuels byproduct streams into value-added products. 3. Optimize industrial processes (such as injection and compression molding) to convert low-value components of various biofuels byproduct streams into value-added products. Methods An injection-molding grade polylactic acid (PLA) was mixed, by weight at room temperature, with distillers dried grains with solubles and glycerol at low shear using a tumbling action to preserve the DDGS fibers� integrity while allowing the glycerol to be absorbed by the grains. Blend compositions were selected based on a DoE (Design of Experiment) approach. DDGS content was 0, 20, 50, and 80% w/w while glycerol content was 0, 5, 10, and 15%; the remainder 10 to 100% was PLA. Each blend was prepared as a 0.91-kg (2-lbm) batch to directly injection mold into tensile and bending bars. The DDGS started in its raw, flaked form, then was de-oiled to remove high-value components and to concentrate the high-fiber portion. The grains were tan in color but slightly darker and coarser than unprocessed DDGS. Glycerol was obtained from a commercial biodiesel facility. Enough blends were molded so that each experimental treatment had three to four replicates. The processing into test specimens involved placing a charge of resin and plasticized DDGS into the hopper of an injection molding machine. The prepared batches were loaded into the hopper, or directly into the throat, of a 250-kN (28-ton), 25 mm (1 in) barrel diameter, screw-auger, hydraulic injection molding machine; the injection pressure was 154 MPa (22,300 psi) and the melt injection speed was 25 mm/sec (1 in/sec). There are four controllable heating zones with the first zone nearest to the hopper and the last one located near the injection nozzle. Temperatures ranged from 188 oC to 200 oC (370 oF to 392 oF) with the higher temperatures in the middle two zones and the lowest at the hopper. The molding cycle time averaged 35 seconds. Each molding cycle produced two test specimens: a tensile bar and a bending bar. The tensile bar was the standard dog-bone shape according to ASTM D638-10 and measured 127 mm (5 in) long, 3.18 mm (1/8 in) thick, and 12.7 mm (1/2 in) in the narrow, break region. The bending specimen measured 127 mm (5 in) long, 3.18 mm (1/8 in) thick, and 12.7 mm (1/2 in) wide and was used in the flexural modulus testing. For the pure PLA case, the bars produced were stiff and clear with some minor fogging. Upon introduction of DDGS, the bars became opaque and dark brown. When broken, the edges of the fracture were slightly rough. As more DDGS was added, the bars became darker and tended to tear when broken. Additionally, under 50% PLA the surface of the bars roughened. For 15/80/5 PLA/DDGS/glycerol some surface cracking and flaking occurred, and the specimens broke easily if flexed. But for the 10/80/10 parts, the cracking was less and no flaking was noticed as the extra 5% glycerol seemed to seal the surface. Additionally, at low PLA levels, a lower fraction of glycerol in the filler tended to produce more jagged, irregular tearing. At glycerol levels of 10% and 15%, the surface had an inherent tackiness to it. Although within the DoE plan, the 20/80/0 mixture could not be successfully processed. (Parts could be formed in the mold but were extremely brittle and broke under the slightest load so part ejection was impossible.) Tensile testing was done in accordance with ASTM D638-10 while the flexular modulus testing was per ASTM D747-10. For conditioning, tensile and bending bars were placed in a constant temperature/humidity chamber for at least 40 hours prior to testing at 23 oC (73.4 oF) and 50% relative humidity, following Procedure A of ASTM D618-08. The reported tensile strength is the maximum stress during the tension test and corresponds to an ultimate tensile strength (UTS). Surface hardness measurements were taken with a Shore-D indentor on tensile and/or bending bars from a total of ten readings (five per side) that were averaged. Results Among the test runs were those for pure PLA, to serve as a baseline, as available in Table 1. Overall, PLA is a moderately strong but typically brittle polymer. Tensile tests revealed the blends to be more brittle with weaker strength. The majority of specimens extended until maximum load and fractured without yielding. When yielding was present, it coincided with the maximum stress, and the break stress was within 5% of the yield. At the same time, the elongation at yield was only a few tenths of a percentage point less than break. Thus for all practical purposes, reporting the tensile strength, tensile modulus, and elongation to break adequately characterizes the material. Table 2 presents the mechanical strength data for the blends which are coded as PLA/DDGS/glycerol content by weight. The averaged data are presented along with one standard deviation. As expected, and consistent with other studies, results indicate that substitution for PLA by plasticized filler significantly reduces tensile stress. Processing up to 90% filler was achieved with the aid of glycerol even though much of the PLA�s original strength is lost. The data show that the reduction in strength is mainly due to the presence of filler rather than the relative amount of glycerol in the filler. This indicates that the plasticizing effect is primarily to aid flow properties and processing; little � or no � bonding between DDGS and PLA occurs at the molecular level. Thus the plasticized grains act as simple filler, surrounded by PLA chains. With any of the blends, the normalized tensile strength is under 50%. It drops to about 40% of the baseline for 20-30% filler. Within the standard deviation, the 35-50% filled PLA had about one-fifth the baseline strength and only one-tenth at the 60-65% level. Over 80% biofiller in specimens exhibited 95% less tensile strength relative to pure PLA along with a large amount of uncertainty in the measurement. There is not a great difference between the 35% to 55% filler strength data even though the relative portion of glycerol is different. On the other hand, the benefit of glycerol as a processing additive is evident when considering the large standard deviation present in the 50/50/0 (no glycerol) data; the 55% to 65% filler indicate weaker strength but far lower measurement uncertainty. Therefore, adding glycerol promotes a more homogenous material with more consistent, but perhaps slightly weaker, mechanical performance. The stiffness of the blends under strain, as measured by tensile (Young�s) modulus exhibits some of the same trending as for strength. DDGS is a stiffer material compared to pure PLA, and the 20% blend (without glycerol) has a 5% greater modulus. With more filler, the presence of plasticized material degrades the tensile modulus with 35% filler yielding about half the baseline stiffness. The 50/50/0 blend recovers much of the original stiffness and exceeds almost all plasticized blends; it has stiffness similar to the 25% plasticized specimens. As with tensile strength, the 50/50/0 material exhibits greater inconsistency among its replicates compared to plasticized runs. Over 50% filler, there is a 70-80% reduction in tensile modulus until only 10% of tensile modulus remains at 90% filler content. While the tensile modulus measures a material�s resistance to deformation under a pulling load, if a bending load is encountered a better measurement of stiffness is the flexural modulus, which combines compressive and tensile effects, and for a polymer may be different in magnitude from its tensile modulus. The flexural modulus qualitatively shows the same trending. However the magnitude of property reduction is much more pronounced as even 20% DDGS-filled PLA suffers a 30% reduction in flexural stiffness. At 55-65% plasticized filler, 80-85% reduction in flex stiffness is found. Lastly, the flexibility and ductility of any material is given in terms of its elongation to break. This is expressed by the percentage that the material strains (stretches) until it fractures. PLA formulations tested are rather brittle with less than 3% elongation, compared to 5% at baseline. The two blends without glycerol are especially brittle with extensions 20% and 30% of the pure PLA for 50% and 20% DDGS, respectively. Otherwise, the presence of plasticization produces a more consistent extension over a range of filler content, regardless of the relative amount of glycerol. Even the 85-90% filled PLA has significant ductility. So the presence of the glycerol adds ductility and provides more binding strength to the DDGS fibers which holds the resin matrix together longer. Normalized surface hardness data indicates that blends up to 60% filler are within 20% of the PLA baseline. With more filler, surface hardness decreases about one-third from pure PLA. The fraction of glycerol in the filler does not affect the hardness as the 50% inclusion level exhibits approximately the same hardness as the 25-30% filled material which has glycerol. Hardness data display smaller measurement uncertainty than the other mechanical properties that were evaluated in this study. Conclusions The present work has demonstrated that glycerol-plasticized DDGS, up to 65% in a PLA matrix, can be injection molded to develop products with adequate mechanical properties. Remaining work continues along the same lines with a thermoplastic starch (TPS). The experiments have been concluded and the data are in the process of being evaluated. Overall, this study is useful because the demand for green products is projected to increase. And data obtained here is valuable to manufacturers that would design with such bio-based materials.
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